1. Field of the Invention
The present invention relates to a solar battery module, particularly to a surface protecting member of the solar battery module which seals the light incident side of a photovoltaic element with a transparent organic polymer resin.
2. Description of the Related Art
In recent years, world wide interest in the preservation of the environment has occurred. In particular, as global warming with carbon dioxide emissions has given rise to serious concerns, the demands for clean energy sources have increased. At present, solar batteries are anticipated as being one of the clean energy sources due to their safety and ease of handling.
There are currently known various types of solar batteries. The following types are typical:
(1) Single crystal silicon solar batteries;
(2) Poly-crystal silicon solar batteries;
(3) Amorphous silicon solar batteries;
(4) Copper-indium selenide solar batteries; and
(5) Compound semiconductor solar batteries.
Among them, thin film polycrystal silicon solar batteries, compound semiconductor solar batteries, and amorphous silicon solar batteries are relatively low in production cost. Research and development have recently been intensively carried out in a variety of areas.
Among these solar batteries, thin film solar batteries, typically amorphous silicon solar batteries where silicon is deposited on a conductive metal substrate and then transparent conductive film is formed thereon, are lightweight and exhibit excellent impact resistance and flexibility. It is apparent that solar batteries have bright future prospects in modular form. However, this type of solar battery must be protected from the environment by covering with a transparent sealing material on the light incident face, which differs from the case where silicon is deposited on a glass substrate.
Conventionally, as surface coating materials, transparent fluoride polymer thin films such as fluorine resin films and fluorine resin paint have been applied to the top surface, and various transparent thermoplastic resins have been used in the interior thereof. Because fluoride polymers have excellent weatherability and water repellent properties, they can reduce efficiency losses of the solar battery module due to decreasing light transmittance from yellowing or cloudiness of deteriorated resin or surface contamination. Further, transparent thermoplastic resins are inexpensive and may be used in great quantities as a filler to protect the photovoltaic element inside the module.
Moreover, to effectively output the generated power, metallic members electrically connect various collecting electrodes in series or in parallel. Each of the connecting members is contained within the solar battery module. The transparent thermoplastic organic resins have the advantage that the resin smooths the surface of the sealing material by removing the irregularities, thereby sealing the electrical connection members such as electrodes and metallic members.
FIG. 6 shows an example of such conventional solar batteries. In FIG. 6, 602 represents a fluoride polymer thin film, 603 represents a transparent thermoplastic organic resin, 601 represents a photovoltaic element, 604 represents an insulating layer, 605 represents a collecting electrode, 606(a) and 606(b) represent a terminal of each element, and 607 represents a metal foil which serially connects the elements via the terminals. The same organic resin used on the front face is also used on the reverse face. Examples of the resin used are fluorine resin films such as ETFE (ethylene-tetrafluoroethylene copolymer) film and PVF (poly vinyl fluoride), which are used as the fluoride polymer thin film; EVA (ethylene-vinyl acetate copolymer) and butyral resin are used as the transparent thermoplastic organic resin; various organic resin films such as nylon film, TEDLAR laminated aluminum foils and the like are used as the insulating layer; gold, silver, soldering materials, conductive pastes and the like are used as the collecting electrodes; and copper, silver, and solder plated copper are used as the metal foil. In example, transparent thermoplastic resin 603 not only functions as an adhesive between the photovoltaic element 601 and the fluorine resin film 602 or the insulating layer 604, but also as a filler for removing the irregularities on the surface of the element and protecting the solar battery from exogenous scratches and impacts.
However, in the conventional structure of the solar battery, the electrical connection members of the photovoltaic element, especially the metallic member, directly contact the transparent thermoplastic organic resin present as the filler. Thus, there is a drawback in that the deterioration of the resin contacting the metallic member is promoted and the metallic member is corroded due to the degradation products of the resin. Degradation of the sealing material often occurs after long-term exposure outdoors. That is, the disadvantages of the conventional structure reside in the promotion of the degradation process catalyzed by the metal and the corrosion of the metal due to acid. These phenomena are especially marked in the case where EVA is used as the transparent thermoplastic organic resin and copper is used as the metallic member. For example, the report of the Jet Propulsion Laboratory for the U.S. Department of Energy, "Flat-Plate Solar Array Project Volume VII: Module Encapsulation (1986)" describes that EVA which contacts copper in the module significantly yellows during 12,000 hours exposure with heating. This is confirmed by our heat resistance test of the module carried out at 150.degree. C., where the yellowing of the EVA on the copper foil is extremely significant compared with other portions of the module.
Furthermore, during our weathering test of the module with a sunshine weathermeter carried out at the same time, a color change of the copper foil to green was observed. This occurrence is apparently due to promotion of the EVA degradation by catalysis of the copper, the corrosion of the copper with acetic acid as the degradation product of EVA, and their synergistic effects.
The above situation is not limited to the yellowing of the EVA on the copper foil. In severe cases, the yellowing of EVA will spread around the copper foil. Because the yellowed sealing material leads to a decreased quantity of light reaching the photovoltaic element, adverse effects such as the decreasing quantity of light which contributes to power generation and decreasing photoelectric conversion efficiency of the solar battery result when the region around the copper foil is a power generating region.
These drawbacks are further promoted when using the integrated module with a roof sheathing where the module temperature rises to a high value. The module temperature in this case is generally around 20.degree. C. higher than in the case of modules having a frame, because there is no air flow across the reverse face. That is, no cooling effect is caused by the wind, different from when the solar battery module is placed on the frame. Such yellowing of the EVA on the copper foil will surely be promoted under this condition.
A different filler resin may be used instead of the EVA resin to avoid these drawbacks. Some resins having high durability such as silicone resin or fluorine resin, for example, have been suggested as substitutes for EVA. However, these resins can not be used due to the high cost of these resins, making it difficult to attain a low cost battery. On the other hand, EVA offers an excellent combination of durability and cost, and presents the best potential sealing material for the solar battery at the present time. Therefore, it would be very difficult to substitute other resins for the EVA.
The electrical connection members of the photovoltaic element are easily damaged due to external mechanical factors. Typically, in the case of a module structure where the top surface is a film as described above, hail impact resistance and scratch resistance of the electrical connection members having irregularities should be improved. A sufficient thickness of the sealing material is required to protect the electrical connection members. Further, the irregularities of the electrical connection members are much greater than the irregularities of the photovoltaic element, so that the electrical connection members including the smooth portions should be filled with great quantities of the resin for protection. This leads to cost increase and weight increase of the module.